The Sunda Arc, which extends trom the eastern Himalayan syntaxis to the Banda Arc, is the zone where the Indian-Australian plate or plates are being subducted beneath the Southeast Asian plate. Major variations occur around this arc, principally as a function of two variables: direction and speed of convergence across the subduction zone, and thickness of sediment on the under-thrusting plate. These variables are manifested in many important variations in the character of the forearc region, including morphology of the trench, morphology and volume of the accretionary prism/outer arc ridge, style and intensity of deformation within the accretionary prism, and tectonic effects on the edge of the overthrusting plate which result from the variabie oblique convergence. The latter includes lateral terrane transport and the opening and closing of marginal basins like the modern Andaman Sea. Understanding these variations and their apparent causes is important for the interpretation of other arcs, regions of terrane accretion, and regions of obducted ophiolites, both extant and preserved as remnants in mountain ranges.